Visual or optical defects which prevent parallel light rays entering the eye from focusing clearly on the retina exist in several varieties. In hyperopia (farsightedness), the point of focus lies behind the retina, generally because the axis of the eyeball is too short. In myopia (nearsightedness), the image is focused in front of the retina, generally because the axis of the eyeball is too long. In astigmatism, refraction is unequal on the different meridians of the eyeball, generally due to asymmetry in the shape of the eye.
Corrective glasses or contact lenses have been used to correct these defects, including convex (plus) lenses for hyperopia, concave (minus) lenses in myopia, and cylindrical lenses in astigmatism. More recently, a surgical technique, myopic or hyperopic keratomileusis has been used to alter cornea curvature and thereby improve refractive error. This method cuts and removes a predicted thickness of the corneal disk with a microkeratome. Additional surgical procedures such as radial keratotomy use microincisions in the cornea to surgically modify the curvature of the cornea and thereby reduce or eliminate myopia or astigmatism.
Photorefractive keratectomy (PRK) uses a laser to ablate the center of the cornea and thus change the cornea. In Automated Lamilar Keratectomy (ALK) pressure is placed on the cornea to bulge the central dome. A flap in the dome is then opened, layers of corneal tissue are removed and the flap is then closed. Procedures combining aspects of ALK/PRK are sometimes used, called LASIK (laser in situ keratectomy).
While these surgical procedures effect long lasting correction of visual defects, they present an inherent risk of permanent damage to a patient's eye. However slight this risk might be, many patients are unwilling to undergo these surgical procedures to correct the curvature of the cornea. Thus, there has existed a need to provide a non-surgical method for reshaping the cornea and thereby effecting correction of visual defects.
As one technique for satisfying such a demand, U.S. Pat. No. 5,695,509 provides an optical contact lens (contact lens) for non-surgically reshaping and altering the curvature of the cornea. When applied to the cornea of a patient, this optical contact lens (hereinafter, sometime referred to as “contact lens”) exerts a selective pressure on the cornea causing displacement of corneal tissue away from a pressure zone to a relief zone, thereby reshaping the patient's cornea and improving the patient's vision without surgical intervention. In general, the design of the optical contact lens induces change in the corneal topography of the patient's eye to make the cornea of a myopic eye more oblate.
This optical contact lens is tooled in response to the specific contour or topography of a patient's cornea and to affect a desired reshaping or correction of the eye's curvatures. When the contact lens is placed on the patient's cornea, a pressure zone of the contact lens exerts a relative selective pressure on the underlying or engaged region of the cornea to effect displacement of corneal tissue away from the region of pressure. A relief zone adjacent to the pressure zone does not contact the cornea and does not exert pressure on the cornea, but is an area where the contact lens is raised above the corneal surface. This area serves to receive corneal tissue which is displaced from the cornea underlying the pressure zone. An anchor zone adjacent to the relief zone and between the relief zone and the periphery of the contact lens controls or guides the reshaping of the corneal tissue, directing displaced tissue to the relief zone. The anchor zone also ensures good centration and maintenance of centration of the contact lens on the cornea thus providing predictability of the result and preventing overshooting the desired correction.
A contact lens of this invention useful in the treatment of myopia contains a central pressure zone, an adjacent annular relief zone, and an annular anchor zone adjacent to the relief zone and located between the relief zone and the periphery of the contact lens. When the contact lens is positioned on the patient's cornea, pressure is exerted by the central pressure zone on the approximate center of the corneal dome, thereby effecting displacement of corneal tissue away from the center of the dome and to the adjacent annular relief area. The pressure exerted at the anchor zone controls reformation of the corneal surface by guiding the displaced tissue into the relief zone. With time, the steep curvature of the myopic eye's corneal dome is flattened or reduced, and light incident over the central cornea will more correctly converge on the retina, thereby improving the patient's vision.
In order to effect the treatment of astigmatism for a patient's corneal dome having one or more curvatures, the contact lens' curvature is arranged in each of given axes to allow the pressure zone to be positioned so as to apply pressure at the steepest meridian, thereby reducing the steep meridian and minimizing or eliminating the difference in curvature. The characteristics of a contact lens for treating astigmatism are similar to those of a contact lens for correcting myopia.
The contact lens provided by the above U.S. Patent can be summarized as follows.
A myopia and/or astigmatism-correcting contact lens for correcting myopia and/or astigmatism based upon the alteration in the shape of a patient's cornea, comprising;                a pressure zone having a first surface defined by the inner surface of the contact lens located on the side of a patient's cornea and positioned at the center of the contact lens, wherein the first surface is formed in a concave shape having a curvature than that of the central surface of the patient's cornea;        a relief zone having a second surface defined by the inner surface of the contact lens located on the side of the patient's cornea and positioned at the periphery of the pressure zone, wherein the second surface is formed in a concave shape; and        an anchor zone having a third surface defined by the inner surface of the contact lens located on the side of the patient's cornea and positioned at the periphery of the relief zone and, wherein the third surface is formed in a concave shape.        
More specifically, in order to induce a specific desired alteration in the shape of the patient's cornea, the first surface has a curvature determined based on the shape of the patient's cornea, and each of the curvatures of the first, second and third surfaces is arranged to satisfy the following formulas,RC=BC+3.00 D (diopter), andAC=BC+0.0−1.0 Dwhere BC is the curvature of the first surface, RC is the curvature of the second surface, and AC is the curvature of the third surface.
The contact lens having the RC and AC arranged as above could achieve some positive results. However, it was significantly effective only for European and American but less effective for Asian. Thus, the inventors have researched the shape of the cornea of Asian, particularly of Japanese, and have found out a desirable curvature of each of the aforementioned surfaces of the contact lens most effective for Asian, particularly for Japanese.